Some printed wiring boards employ a core substrate on which a pair of differential signal wirings is patterned. When the core substrate is fabricated by impregnating a glass fiber cloth, made up of warp yarns and weft yarns, with a resin, a delay in signal transmission in the core substrate due to the positional relationship between projected/recessed positions of the glass fiber cloth and the differential signal wirings may not cause a significant problem at a transmission frequency of 1.6 GHz or below, for example.
Meanwhile, in a case where the transmission frequency increases up to a range of 3 to 5 GHz, a delay in signal transmission may be generated with a difference at a level that is not negligible, or an impedance mismatch may be caused. To cope with the difference in the delay time and the impedance mismatch, a technique has been proposed which provides, e.g., a structure for adjusting the positional relationship between the projected/recessed positions of the glass fiber cloth and the differential signal wirings with more consideration paid to the projected/recessed positions of the glass fiber cloth.
In the stage where a differential signal wiring pattern is drawn, however, the surface of the substrate is covered with a copper foil and the glass fiber cloth in the substrate cannot be visually recognized. Also, even in the substrate (prepreg) in the stage where the copper foil is not yet coated over the surface of the substrate, it is difficult to visually recognize individual yarns (warp yarns and weft yarns) of the glass fiber cloth for the reason that the glass fiber cloth is impregnated with the resin. Recently, spread-type glass fibers, i.e., glass fibers collapsed to spread yarns (each made of the glass fibers) laterally, have often been used. In the case of the substrate using the spread-type glass fibers, a basket hole, i.e., a gap between individual yarns of a glass fiber cloth, is so very small as to further increase a difficulty in visual recognition of the yarns. Stated another way, it has become more difficult to recognize the individual yarns of the glass fiber cloth one by one.
On the other hand, it has been proposed to change respective colors of warp yarns and weft yarns for the purposes of evaluating distortions of fibers or identifying the properties of the fibers. It has also been proposed to employ a reinforced fabric of a structure obtained by stacking two fabrics, each being made up of warp yarns and weft yarns and each having a front texture and a rear texture, in such a state that respective orientations of the front texture and the rear texture are turned upside down and the positional relationship between the warp yarns and the weft yarns are further reversed. However, an improvement in signal transmission is not taken into consideration at all. Similar structures to the above-described printed circuit board are disclosed in JP-A-5-39372, JP-A-11-286847, and JP-A-2006-233369, for example.